[QE-users] Question regarding mixing of PBE and PBESOL PP

[Yeon, Jejoon]

Hello


I ran several DFTs few times before, but I only have limited amount of knowledge and experience in DFTs.


I have Pyrope (Mg3Al2Si3O12) and Grossular (Ca3Al2Si3O12) unit cell, and wish to preform relax and vc-relax. After I get the relaxed atomic structure and cell size, I wish to make an energy curve of Pyrope / Grossular structure w.r.t. compression and expansion of volume, using expanded / compressed unit cells. Purpose of this DFT is to get the database for the parametrization of empirical force field, especially for mechanical properties like stress-strain curve and elastic constants.


Now, I have PBE-PAW(kjpaw) and PBESOL-PAW(kjpaw) PPs for all atom types. I have no problem about Pyrope, I can run it with PBESOL. But Grossular is an issue. For Ca, PBE is only available option from QE PP webpage, there is no PBESOL for Ca yet. Accordingly, if I want to calculate energies of Grossular structure, I have three options as far as I know:

1) input_dft = 'PBE', while mix the PBE (Ca) and PBESOL (Al, Si, O)

2) inout_dft = 'PBESOL', while mix the PBE (Ca) and PBESOL (Al, Si, O)

3) just use PBE PPs for all 4 atom types.



At this point, I'm not sure what would be the good choice for my purpose. I wish to use PBESOL because it is known to have better prediction for solid properties. But I'm not sure if I can mix them with PBE, and if I mix, which input_dft option should I need to choose.


I ran test relaxations for 3 cases, and I found that the final atom coordinates of all 3 cases are very similar, they are mostly the same up to second decimal point numbers. Their final optimized energies are different, but as far as I know I can't compare them, isn't it? I'm not sure on which criteria should I need compare and select the best one from those three options.


Or, if anyone had a similar experience, I welcome any advises or suggestions.


Just in case, following is my example in file.


&CONTROL
  prefix="03_Relax_Grossular_80_800_PBESOL",
  calculation='relax',
  outdir="/scratch/jj/20190501_QEDFT_Grossular_EoS/03_Relax_Grossular_80_800_PBESOL/tmp",
  pseudo_dir="/home/QE_pseudo/",
  restart_mode= 'from_scratch',
  nstep = 1000
/

&SYSTEM
  ibrav=0,
  nat=20,
  ntyp=4,
  ecutwfc=80,
  ecutrho=800,
  occupations='smearing',
  smearing='methfessel-paxton',
  degauss=0.005000,
  input_dft='PBESOL'
/

&ELECTRONS
  diagonalization='cg',
  conv_thr=1d-07,
  mixing_mode='plain',
  mixing_beta=0.100,
/

&IONS
  ion_dynamics = 'bfgs',
/

ATOMIC_SPECIES
  Ca 40.078000 Ca.pbe-spn-kjpaw_psl.1.0.0.UPF
  Al 26.981539 Al.pbesol-n-kjpaw_psl.1.0.0.UPF
  Si 28.085500 Si.pbesol-n-kjpaw_psl.1.0.0.UPF
  O 15.999400 O.pbesol-n-kjpaw_psl.1.0.0.UPF

ATOMIC_POSITIONS {angstrom}
Ca    1.480750    0.000000    2.961500
Ca    2.961500    1.480750    0.000000
Ca    0.000000    2.961500    1.480750
Al    0.000000    0.000000    0.000000
Al    2.961500    2.961500    2.961500
Si    0.000000    2.961500    4.442250
Si    4.442250    0.000000    2.961500
Si    2.961500    4.442250    0.000000
O     0.450148    5.393484    1.791116
O     1.791116    0.450148    5.393484
O     5.393484    1.791116    0.450148
O     3.491016    2.511352    4.752616
O     2.511352    4.752616    3.491016
O     4.752616    3.491016    2.511352
O     5.472852    0.529516    4.131885
O     4.131885    5.472852    0.529516
O     0.529516    4.131885    5.472852
O     2.431984    3.411648    1.170385
O     3.411648    1.170385    2.431984
O     1.170385    2.431984    3.411648

K_POINTS {automatic}
  4 4 4 0 0 0

CELL_PARAMETERS {angstrom}
   5.923000000   0.000000000   0.000000000
   0.000000000   5.923000000   0.000000000
   0.000000000   0.000000000   5.923000000


Thank you


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